Compound and thermoelectric conversion material

ABSTRACT

The present invention relates to a compound containing at least germanium, tellurium, bismuth, copper, antimony and silver as constituent elements.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Section 371 of International Application No.PCT/JP2017/013362, filed Mar. 30, 2017, which was published in theJapanese language on Oct. 5, 2017 under International Publication No. WO2017/170911 A1, the disclosure of which is incorporated herein byreference in its entirety.

TECHNICAL FIELD

The present invention relates to a compound and a thermoelectricconversion material.

Priority is claimed on Japanese Patent Application No. 2016-073745,filed Mar. 31, 2016, and Japanese Patent Application No. 2016-177049,filed Sep. 9, 2016, the contents of which are incorporated herein byreference.

BACKGROUND ART

Thermoelectric conversion devices that utilize the Seebeck effect areable to convert thermal energy into electrical energy. In other words,by using a thermoelectric conversion device, electric power can beobtained directly from thermal energy. For example, by using athermoelectric conversion device, the waste heat from the engine of anautomobile can be recovered, and a portion of that waste heat can thenbe converted to electric power. For example, waste heat from a factorycan be recovered, and a portion of that waste heat converted to electricpower.

In recent years, from the viewpoints of suppressing the consumption ofenergy resources and the emission of carbon dioxide, improvements inenergy efficiency continue to be demanded ever more strongly. As aresult, much research is being conducted with the aim of improving theperformance of thermoelectric conversion.

The thermal efficiency and output characteristics obtained from athermoelectric conversion device are limited by the performance of thethermoelectric conversion material that constitutes the thermoelectricconversion device. Consequently, much investigation is being undertakeninto improving the performance of thermoelectric conversion materials.

Examples of heat sources having a high temperature range include thewaste heat from automobiles and the waste heat from factories. Becausethe temperature of these waste heat sources is near 500° C.,investigation of thermoelectric conversion materials that operate fromroom temperature to near 500° C. is being actively pursued. Examples ofmaterials that exhibit a high thermoelectric figure of merit and highoutput factor in the temperature region near 500° C., and also have heatresistance, include thermoelectric conversion materials containing agermanium telluride-based compound.

Here, the thermal efficiency of a thermoelectric conversion material isrepresented by formulas shown below. The maximum thermal efficiencyη_(opt) obtainable using a thermoelectric conversion material isrepresented by a formula (1) shown below. Here, in formula (1) shownbelow, T_(H) represents the temperature at the high temperature end[units: K], T_(C) represents the temperature at the low temperature end[units: K], T_(ave) represents the average of T_(H) and T_(C) [units:K], and Z represents the average thermoelectric figure of merit [1/K] ofthe thermoelectric conversion material in the temperature range.

$\begin{matrix}\left\lbrack {{Numerical}\mspace{14mu}{formula}\mspace{14mu} 1} \right\rbrack & \; \\{\eta_{opt} = {\frac{T_{H} - T_{C}}{T_{H}}\frac{\sqrt{1 + {ZT}_{ave}} - 1}{\sqrt{1 + {ZT}_{ave}} + \frac{\tau_{C}}{\tau_{H}}}}} & (1)\end{matrix}$

The thermoelectric figure of merit z [1/K] of a thermoelectricconversion material at a given temperature T is represented by a formula(2) shown below. Here, α [V/K], ρ [Ωm], and κ [W/(mK)] represent theSeebeck coefficient, the resistivity, and the thermal conductivityrespectively of the thermoelectric conversion material at a giventemperature T.

$\begin{matrix}\left\lbrack {{Numerical}\mspace{14mu}{formula}\mspace{14mu} 2} \right\rbrack & \; \\{{zT} = {\frac{a^{2}}{\rho\;\kappa}T}} & (2)\end{matrix}$

A larger value for the physical property zT of the thermoelectricconversion material indicates a higher value for the maximum thermalefficiency η_(opt) obtainable by thermoelectric conversion. The averageof the thermoelectric figure of merit z of the thermoelectric conversionmaterial in the temperature range that is used is deemed thethermoelectric figure of merit Z. In order to increase the thermalefficiency, it is desirable to obtain a high thermoelectric figure ofmerit z across a broad temperature range.

The output factor (also known as the power factor) [W/(mK²)] is used asan indicator of the maximum output obtainable at a temperature T using athermoelectric conversion material. The power factor is represented by aformula (3) shown below. In the formula (3) below, α [V/K] representsthe Seebeck coefficient of the thermoelectric conversion material at agiven temperature T, and ρ[Ωm] represents the resistivity.

$\begin{matrix}\left\lbrack {{Numerical}\mspace{14mu}{formula}\mspace{14mu} 3} \right\rbrack & \; \\{{{Power}\mspace{14mu}{Factor}} = \frac{a^{2}}{\rho}} & (3)\end{matrix}$

A higher power factor for the thermoelectric conversion material asrepresented by the above formula (3) indicates a larger maximum outputobtainable by thermoelectric conversion.

Examples of heat sources having a temperature range near 500° C. includethe waste heat from automobiles and the waste heat from factories, andinvestigation of thermoelectric conversion materials that operate fromroom temperature to near 500° C. is being actively pursued. Examples ofmaterials that exhibit a high thermoelectric figure of merit and highpower factor in this temperature range, and also have heat resistance,include thermoelectric conversion materials containing a germaniumtelluride-based compound.

Patent Document 1 discloses that a thermoelectric conversion materialproduced by jointly grinding and mixing metal powders composed ofgermanium, tellurium, silver and antimony, and then performing moldingand sintering by hot isostatic pressing, or produced by jointlygrinding, mixing and calcining the metal powders, and then performingmolding and sintering, exhibits excellent thermoelectric conversioncharacteristics at 700 K.

PRIOR ART LITERATURE Patent Documents

-   Patent Document 1: JP H06-169110 A

SUMMARY OF INVENTION Problems to be Solved by the Invention

When performing thermal recovery of waste heat from automobiles andfactories, and then using that waste heat as electric power, it isdesirable that a large output is obtained as electric power, and thatthe energy efficiency is high. In order to achieve these effects, it isdesirable that the compound that constitutes the thermoelectricconversion device has a high power factor and a high zT value.

The present invention has been developed in light of thesecircumstances, and has objects of providing a compound having a highpower factor and a high zT value in the high-temperature region near500° C., a thermoelectric conversion material containing the compound,and a method for producing the compound.

Means for Solving the Problems

As a result of intensive investigation, the inventors of the presentinvention developed the present invention. In other words, the presentinvention has the following aspects.

[1] A compound containing at least germanium, tellurium, bismuth,copper, antimony and silver as constituent elements.

[2] The compound according to [1], having a carrier density of not morethan 1.0×10²¹ cm⁻³.

[3] The compound according to [1] or [2], represented by a chemicalformula Ge_(1+a-b-c-d-e)Bi_(b)Cu_(c)Sb_(d)Ag_(e)Te (wherein−0.05≤a≤0.10, 0≤b≤0.10, 0≤c≤0.10, 0≤d≤0.20, and 0≤e≤0.20).

[4] The compound according to any one of [1] to [3], wherein anintensity ratio I(Ge)/I(GeTe) between a maximum intensity I(GeTe) of anXRD peak attributable to germanium telluride and a maximum intensityI(Ge) of an XRD peak attributable to germanium metal is not more than0.025.[5] The compound according to any one of [1] to [4], wherein the longestaxis of bismuth, copper, antimony or silver, which are ubiquitous withinthe compound, is less than 2.0 μm.[6] The compound according to any one of [1] to [4], wherein the longestaxes of bismuth, copper, antimony and silver, which are ubiquitouswithin the compound, are all less than 2.0 μm.[7] The compound according to any one of [1] to [4], wherein the longestaxis of crystals of bismuth, copper, antimony or silver within thecompound is less than 2.0 μm.[8] The compound according to any one of [1] to [4], wherein the longestaxes of crystals of bismuth, copper, antimony and silver within thecompound are all less than 2.0 μm.[9] A thermoelectric conversion material containing the compoundaccording to any one of [1] to [8].

Effects of the Invention

The present invention is able to provide a compound having a high powerfactor and a high zT value in the high-temperature region near 500° C.,a thermoelectric conversion material containing the compound, and amethod for producing the compound.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view schematically illustrating a thermoelectricconversion module.

FIG. 2 is a diagram illustrating the composition distribution of thecompound of Example 1.

FIG. 3 is a diagram illustrating the composition distribution of thecompound of Example 2.

FIG. 4 is a diagram illustrating the composition distribution of thecompound of Comparative Example 1.

FIG. 5 is a series of diagrams illustrating various thermoelectricconversion property values relative to temperature for the compounds ofExamples 1 and 2 and Comparative Example 1.

FIG. 6 is a perspective view illustrating the basic structure of athermoelectric conversion module.

FIG. 7 is an exploded top view of the thermoelectric conversion module.

FIG. 8A is a cross-sectional schematic view along the line VIII-VIII inFIG. 7.

FIG. 8B is a cross-sectional schematic view along the line VIII-VIII inFIG. 7.

FIG. 9A is a diagram schematically illustrating a thermoelectricconversion device.

FIG. 9B is a diagram schematically illustrating a thermoelectricconversion device.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

<Compound>

The compound of the present invention is characterized by containing atleast germanium, tellurium, bismuth, copper, antimony and silver asconstituent elements. It is more preferable that the compound is agermanium telluride doped with bismuth, copper, antimony and silver. Inother words, a compound in which bismuth, copper, antimony and silverare subjected to solid dissolution in germanium telluride, and arearranged within the crystal lattice of, or between the atoms of, thegermanium telluride.

Conventional compounds composed of germanium telluride doped with silverand antimony are known to have high carrier densities. In the case ofcompounds used in thermoelectric conversion applications, in order toincrease the Seebeck coefficient within the range in which theresistivity does not become too large, thereby increasing the outputcharacteristics and zT, it is preferable that the carrier density islow.

In the present invention, by doping the germanium telluride with copperand bismuth in addition to silver and antimony, the carrier density canbe optimized.

Accordingly, when the compound of the present invention is included as athermoelectric conversion material, a high power factor and high zTvalue can be realized.

[Material Properties]

The compound in the present invention contains at least germanium,tellurium, bismuth, copper, antimony and silver as constituent elements.

[Carrier Density]

In the compound of the present invention, in order to increase theSeebeck coefficient within the range in which the resistivity does notbecome too large, thereby increasing the output characteristics and zT,the carrier density at a temperature of 10 K is preferably not more than1.0×10²¹ cm⁻³. The carrier density is more preferably from 1.0×10¹⁹ cm⁻³to 1.0×10²¹ cm⁻³, and even more preferably from 5.0×10¹⁹ cm⁻³ to7.0×10²⁰ cm⁻³. In this description, the term carrier refers to electronsand positive holes. The carrier density indicates the abundance ofelectrons or holes in the compound per unit of volume.

For example, five-terminal Hall measurement with a physical propertymeasurement system PPMS (manufactured by Quantum Design, Inc.) and aspecial-purpose DC resistance sample pack can be used for measuring thecarrier density of a sample. The Hall measurement can be performed bystabilizing the sample temperature, applying a magnetic fieldperpendicularly to the sample surface, and measuring the Hallresistance. The Hall coefficient is calculated from the slope of theHall resistance relative to the magnetic field, and the carrier densityis then calculated from the Hall coefficient. At temperatures near roomtemperature, noise can sometimes cause hysteresis in the Hallresistance, and therefore measurement is preferably performed at lowtemperature.

In order to adjust the carrier density and adjust the phonon diffusionlength to increase the thermoelectric conversion characteristics of zTand the power factor, an element besides germanium, tellurium, bismuth,copper, antimony and silver may also be added. The compound may includeany one of these added elements, or may include two or more elements.The amount of each added element in the compound is, independently,typically not more than 0.2 mol, and more preferably not more than 0.1mol, per 1 mol of tellurium within the compound. In another aspect ofthe present invention, the amount of each added element in the compoundis, independently, preferably from 0.005 to 0.2 mol, and more preferablyfrom 0.01 to 0.1 mol, per 1 mol of tellurium within the compound.

[Composition of the Compound]

The compound of the present invention is characterized by containinggermanium, tellurium, bismuth, copper, antimony and silver within thecomposition.

In order to suppress segregation in the composition, and increase zT andthe power factor, the compound preferably has a composition with a rangerepresented by a chemical formulaGe_(1+a-b-c-d-e)Bi_(b)Cu_(c)Sb_(d)Ag_(e)Te, wherein −0.05≤a≤0.10,0≤b≤0.10, 0≤c≤0.10, 0≤d≤0.20, and 0≤e≤0.20. It is more preferable that0≤a≤0.05, 0.02≤b≤0.10, 0.02≤c≤0.10, 0.05≤d≤0.20, and 0.05≤e≤0.20. Inthis description, segregation refers to a state wherein, when a melt ofa metal or alloy composed of a plurality of elements solidifies, aportion of the elements dispersed within the metal or alloy crystallize,and also refers to those crystals.

[Intensity Ratio of Powder X-Ray Diffraction Peaks for Compound]

The intensity ratio I(Ge)/I(GeTe) between the maximum intensity I(GeTe)of an XRD peak attributable to germanium telluride and the maximumintensity I(Ge) of an XRD peak attributable to germanium metal can becalculated from a powder X-ray diffraction pattern.

In the present invention, I(Ge)/I(GeTe) is preferably not more than0.025. In order to obtain a high zT value and high outputcharacteristics across a broader temperature range, I(Ge)/I(GeTe) ismore preferably less than 0.015, and it is even more preferable that anXRD peak attributable to germanium metal cannot be detected, namely thatI(Ge)/I(GeTe) is 0.

[Compound Crystal Structure]

The crystal structure of the compound can be analyzed, for example,based on the powder X-ray diffraction pattern obtained using a powderX-ray diffractometer. Germanium telluride is known to have two types ofcrystal forms, namely α-GeTe which has a rhombohedral structure of theR3m space group, and γ-GeTe which has a cubic structure of the Fm-3mspace group. The compound of the present invention preferably has anα-GeTe crystal structure having a rhombohedral structure of the R3mspace group. It is even more preferable that the compound of the presentinvention contains no γ-GeTe having a cubic structure of the Fm-3m spacegroup, and has only the α-GeTe crystal structure having a rhombohedralstructure of the R3m space group.

[Composition Distribution of Compound]

The ubiquity of the composition within the compound can be analyzed froma composition distribution diagram of a sample of the compound obtained,for example, using a scanning electron microscope (hereinafter sometimesabbreviated as SEM) fitted with an energy dispersive X-ray spectrometer(hereinafter sometimes abbreviated as EDX). In this description,ubiquity refers to the distribution of each constituent element withinthe compound.

In the present invention, analysis is performed under conditions inwhich the composition distribution of bismuth crystals, copper crystals,antimony crystals and silver crystals of 0.2 μm or larger can be clearlyidentified. Conditions in which the composition distribution of bismuthcrystals and copper crystals of 0.2 μm or larger can be clearlyidentified include the conditions described in the following examples.In this description, the “longest axis” can be calculated from the SEMimage, and means the longest axis among any arbitrary axis (length) in atwo-dimensional cross section of any of the ubiquitous bismuth crystals,copper crystals, antimony crystals and silver crystals that appear inthe SEM image.

In the present invention, the longest axis of the bismuth crystals,copper crystals, antimony crystals and/or silver crystals which areubiquitous within the compound is preferably less than 2.0 μm. In orderto obtain a high zT value and high output characteristics across abroader temperature range, the longest axis of the bismuth crystals,copper crystals, antimony crystals and/or silver crystals which areubiquitous within the thermoelectric conversion material is morepreferably not more than 1 μm, and even more preferably 0.5 μm or less.

In another aspect of the present invention, the longest axis of thebismuth crystals, copper crystals, antimony crystals and/or silvercrystals which are ubiquitous within the compound is preferably at least0.001 μm but less than 2.0 μm. In order to obtain a high zT value andhigh output characteristics across a broader temperature range, thelongest axis of the bismuth crystals, copper crystals, antimony crystalsand/or silver crystals which are ubiquitous within the thermoelectricconversion material is more preferably at least 0.002 μm but not morethan 1 μm, and even more preferably at least 0.005 μm but not more than0.5 μm.

In other words, the compound described above is characterized by thebismuth crystals, copper crystals, antimony crystals or silver crystalsexisting uniformly throughout the compound, with little segregation ofthe elements. As a result, when the compound is included in athermoelectric conversion material, a high power factor and zT value canbe realized.

The longest axes of the bismuth crystals, the copper crystals, theantimony crystals and the silver crystals may be of the same length, orof different lengths, provided they are each less than 2.0 μm. In thosecases where the longest axis differs for the bismuth crystals, thecopper crystals, the antimony crystals and the silver crystals, thedifference between the longest axis of the element for which the longestaxis of the crystals is longest and the longest axis of the element forwhich the longest axis of the crystals is shortest is preferably greaterthan 0 μm but less than 1.0 μm, more preferably greater than 0 μm butnot more than 0.5 μm, and even more preferably greater than 0 μm but notmore than 0.25 μm.

[Crystal Domains]

It is preferable that the compound of the present invention has stripedcrystal domains within the crystals. A crystal domain refers to a regionwhere crystals are oriented in the same direction. Crystal domains canbe observed, for example, in a transmission electron microscope(hereinafter sometimes abbreviated as TEM) image obtained using a TEM.In the present invention, it is preferable that a plurality of stripedcrystal domains are observed. In this description, the width of acrystal domain refers to the width of a dark portion or a light portionobserved as a stripe in the TEM image, whereas the length of a crystaldomain refers to the length of the dark portion or light portion. Thewidth of a crystal domain is shorter than the length, and the width ispreferably at least 0.005 μm but not more than 1 μm. The length ispreferably at least 0.05 μm, and more preferably 0.1 μm or greater. Inanother aspect of the present invention, the length of a crystal domainis preferably at least 0.05 μm but not more than 50 μm, and is morepreferably at least 0.1 μm but not more than 20 μm.

<Thermoelectric Conversion Material>

A thermoelectric conversion material of the present invention maycontain the compound of the present invention described above as amaterial. The compound of the present invention represents the maincomponent, but small amounts of other additives may also be included.The amount of the first embodiment or second embodiment described abovein the thermoelectric conversion material is typically from 50% to 100%,preferably from 70% to 100%, more preferably from 80% to 100%, and evenmore preferably from 90% to 100%. Provided the amount of the firstembodiment or second embodiment in the thermoelectric conversionmaterial falls within the above range, excellent thermoelectricconversion characteristics are displayed.

Aspects of the present invention include the thermoelectric conversionmaterials described below.

<1> A thermoelectric conversion material containing at least germanium,tellurium, bismuth, copper, antimony and silver as constituent elements,wherein

zT at 200° C. is at least 0.60,

zT at 300° C. is at least 1.00,

zT at 400° C. is at least 1.20, and

zT at 500° C. is at least 1.40.

<2> A thermoelectric conversion material containing at least germanium,tellurium, bismuth, copper, antimony and silver as constituent elements,wherein

a ratio P.F.(500° C.)/P.F.(300° C.) between the P.F.(500° C.) at 500° C.and the P.F.(300° C.) at 300° C. is not more than 1.05, and

a ratio zT(500° C.)/zT(300° C.) between the zT(500° C.) at 500° C. andthe zT(300° C.) at 300° C. is not more than 1.40.

zT is the product of the thermoelectric figure of merit z [1/K] of thethermoelectric conversion material at a given temperature T, and thetemperature T [K]. P.F. is an abbreviation for the power factor[ρW/(cmK²)] which is output factor.

Other aspects of the present invention include the thermoelectricconversion methods described below.

<1> A thermoelectric conversion method wherein thermoelectric conversionis performed by applying heat to one end of a compound containing atleast germanium, tellurium, bismuth, copper, antimony and silver asconstituent elements, thereby generating a temperature difference in thecompound with the one end functioning as the high temperature side andthe other end functioning as the low temperature side.<2> The thermoelectric conversion method according to <1>, wherein thecarrier density of the compound is not more than 1.0×10²¹ cm⁻³.

The thermoelectric conversion material of the present invention is amaterial which contains the compound of the present invention describedabove, and has thermoelectric conversion properties, and is a materialthat is used for forming a thermoelectric conversion element provided ina thermoelectric conversion device. In this description, athermoelectric conversion element is an element that utilizes theSeebeck effect or the like to convert thermal energy into electricalenergy. A thermoelectric conversion element for which the Seebeckcoefficient is positive is termed a p-type thermoelectric conversionelement, and a thermoelectric conversion element for which the Seebeckcoefficient is negative is termed an n-type thermoelectric conversionelement. Here, thermoelectric conversion properties mean properties thatconvert thermal energy to electrical energy using the Seebeck effect, athermomagnetic effect, or the spin Seebeck effect or the like.

<Thermoelectric Conversion Device>

A thermoelectric conversion device generally contains a p-typethermoelectric conversion element, an n-type thermoelectric conversionelement, and metal electrodes. A thermoelectric conversion deviceillustrated in FIG. 9A has a p-type thermoelectric conversion element12, an n-type thermoelectric conversion element 13, and metal electrodes15, 16 and 17.

The thermoelectric conversion mechanism of the thermoelectric conversiondevice is described using FIG. 9A and FIG. 9B. In FIG. 9A, thethermoelectric conversion device 21 has the p-type thermoelectricconversion element 12, the n-type thermoelectric conversion element 13,a first metal electrode 15, a second metal electrode 16, and a thirdmetal electrode 17. The p-type thermoelectric conversion element 12 isdisposed between the first metal electrode 15 and the third metalelectrode 17. The n-type thermoelectric conversion element 13 isdisposed between the first metal electrode 15 and the second metalelectrode 16. A heat source 41 may be disposed on the surface of thefirst metal electrode 15 opposite the surface that is connected to thep-type thermoelectric conversion element 12 and the n-typethermoelectric conversion element 13. A heat radiating plate 42 may bedisposed on the surface of the second metal electrode 16 opposite thesurface that is connected to the n-type thermoelectric conversionelement 13, and the surface of the third metal electrode 17 opposite thesurface that is connected to the p-type thermoelectric conversionelement 12. For example, waste heat from an automobile or waste heatfrom a factory can be used as the heat source 41.

The heat source 41 transmits heat to the upper portions of the p-typethermoelectric conversion element 12 and the n-type thermoelectricconversion element 13 via the first metal electrode 15. On the otherhand, heat from the lower portion of the n-type thermoelectricconversion element 13 is radiated from the heat radiating plate via thesecond metal electrode 16, and heat from the lower portion of the p-typethermoelectric conversion element 12 is radiated from the heat radiatingplate via the third metal electrode 17. As a result, a temperaturegradient develops between the upper portions and the lower portions ofthe p-type thermoelectric conversion element 12 and the n-typethermoelectric conversion element 13.

Positive holes (h+) bearing a positive charge in the p-typethermoelectric conversion element 12 move from the higher temperatureupper end portion to the lower temperature lower end portion, therebygenerating a thermoelectromotive force. On the other hand, electrons(e−) bearing a negative charge in the n-type thermoelectric conversionelement 13 move from the higher temperature upper end portion to thelower temperature lower end portion, thereby generating athermoelectromotive force. Because the potential differences of thep-type thermoelectric conversion element 12 and the n-typethermoelectric conversion element 13 are opposite, by electricallyconnecting the upper end portions of the two elements via the firstmetal electrode 15 as illustrated in FIG. 9A, the electromotive forcebetween an electrode 43 and an electrode 44 becomes the sum of thethermoelectromotive force of the p-type thermoelectric conversionelement 12 and the thermoelectromotive force of the n-typethermoelectric conversion element 13. In this example, the 43 sidebecomes the negative electrode and the 44 side becomes the positiveelectrode.

A thermoelectric conversion device 21′ illustrated using FIG. 9B has asimilar structure to FIG. 9A, but an external load 45 is connectedbetween an electrode 43′ and an electrode 44′. Examples of the externalload 45 include a part of an electrical device, meaning a current can beprovided to the electrical device. Examples of the electrical deviceinclude a battery, a capacitor, or a motor or the like.

The p-type thermoelectric conversion element and the n-typethermoelectric conversion element provided in the thermoelectricconversion device can each be obtained, for example, by mechanicallyprocessing a thermoelectric conversion material having p-type or n-typeelectronic properties into the desired shape.

The thermoelectric conversion material of the present inventiondescribed above can be used as the thermoelectric conversion materialhaving p-type or n-type electronic properties. In other words, thethermoelectric conversion element may contain a processed product of thethermoelectric conversion material of the present invention.

The thermoelectric conversion element may form a layered structure, andfor example, may have a layer formed from the thermoelectric conversionmaterial of the present invention, and another layer. Examples of theother layer include a connection layer or a barrier layer.

The layer formed from the thermoelectric conversion material can beobtained by mechanically processing the compound of the presentinvention into a desired shape as the thermoelectric conversionmaterial.

The thermoelectric conversion element may have a connection layerbetween the layer formed from the thermoelectric conversion material inthe thermoelectric conversion element and a metal electrode. Byincluding a connection layer in the thermoelectric conversion element,the thermoelectric conversion element and the metal electrode can beconnected more favorably electrically and mechanically. As a result, thecontact resistance between the thermoelectric conversion element and themetal electrode can be reduced. Examples of joint materials for formingthe connection layer include elements that increase the carrier density,and specific examples include silver, gold, and platinum and the like.Although there are no particular limitations on the thickness of theconnection layer, the thickness is preferably from 0.001 to 20 μm, andmore preferably from 0.005 to 10 μm.

The thermoelectric conversion element may have a barrier layer betweenthe layer formed from the thermoelectric conversion material in thethermoelectric conversion element and a metal electrode. By including abarrier layer, reaction caused by contact between the thermoelectricconversion material in the thermoelectric conversion element and themetal electrode can be prevented. In those cases where thethermoelectric conversion element has a connection layer describedabove, the thermoelectric conversion element may have a barrier layerbetween the layer formed from the thermoelectric conversion material andthe connection layer in the thermoelectric conversion element. Byincluding a barrier layer in the thermoelectric conversion element,reaction caused by contact between the thermoelectric conversionmaterial and the connection layer in the thermoelectric conversionelement can be prevented. Examples of the material for forming thebarrier layer include elements that have the effect of preventingmovement of at least one element contained in the layer formed from thethermoelectric conversion material, the connection layer or the metalelectrode. Specific examples of such elements include aluminum,titanium, chromium, iron, cobalt, nickel, copper, zinc, molybdenum,silver and tantalum. Although there are no particular limitations on thethickness of the barrier layer, the thickness is preferably from 0.5 to100 μm, and more preferably from 0.1 to 50 μm.

In order to prevent reaction between the thermoelectric conversionmaterial and gases in the environment in which the thermoelectricconversion element is placed, or prevent the diffusion of substancesthat may be generated from the thermoelectric conversion material, thethermoelectric conversion element may have a protective film on thesurface of the layer formed from the thermoelectric conversion materialthat is able to contact the gases. Examples of the material for theprotective film include compounds containing silicon. Although there areno particular limitations on the thickness of the protective film, thethickness is preferably from 0.5 to 100 μm, and more preferably from 1to 50 μm.

A thermoelectric conversion module is a module formed by combining aplurality of thermoelectric conversion devices into a single unit. Inother words, a thermoelectric conversion module has a plurality ofthermoelectric conversion devices.

One aspect of the present invention is a thermoelectric conversionmodule described below.

A thermoelectric conversion module having:

a plurality of p-type thermoelectric conversion elements,

a plurality of n-type thermoelectric conversion elements, and

a plurality of metal electrodes, wherein

the plurality of p-type thermoelectric conversion elements and theplurality of n-type thermoelectric conversion elements are connectedelectrically in series in an alternating manner via the plurality ofelectrodes, and

the plurality of p-type thermoelectric conversion elements and theplurality of n-type thermoelectric conversion elements contain at leastgermanium, tellurium, bismuth, copper, antimony and silver asconstituent elements.

One example of a thermoelectric conversion module is described usingFIG. 1 and FIGS. 6 to 8.

As illustrated in the perspective view of FIG. 6, which shows the basicstructure of the thermoelectric conversion module, a plurality ofthermoelectric conversion elements 11 are disposed in a lattice-likearrangement in the thermoelectric conversion module. Insulating plates18 for reinforcing the thermoelectric conversion module 20 are installedon the top and bottom of the thermoelectric conversion module.

As illustrated in the exploded top view of the thermoelectric conversionmodule shown in FIG. 7, p-type thermoelectric conversion elements 12 andn-type thermoelectric conversion elements 13 are arranged in analternating two-dimensional array in the thermoelectric conversionmodule 20. All of the p-type thermoelectric conversion elements 12 andthe n-type thermoelectric conversion elements 13 are connectedelectrically in series from a lead wire 31 to a lead wire 32, asillustrated by a two-dot chain line in the figure. As illustrated inFIG. 1, which is a side view schematically illustrating thethermoelectric conversion module, and FIG. 8A and FIG. 8B, which arecross-sectional views along the line VIII-VIII in FIG. 7, all of thep-type thermoelectric conversion elements 12 and the n-typethermoelectric conversion elements 13 in the thermoelectric conversionmodule 20 are connected electrically in series via metal electrodes 14.The connection illustrated by the two-dot chain line in FIG. 7 is oneexample, and although there are no particular limitations on the mannerin which the elements are connected, it is preferable that all of thep-type thermoelectric conversion elements 12 and the n-typethermoelectric conversion elements 13 are connected electrically inseries in an alternating manner via metal electrodes. By connecting thelead wires 31 and 32 to the metal electrodes that are connected to thep-type thermoelectric conversion element and the n-type thermoelectricconversion element that are positioned at the two ends of the pluralityof electrically series-connected p-type thermoelectric conversionelements 12 and n-type thermoelectric conversion elements 13, externaloutput can be achieved.

Conventional lead wires can be used as the above lead wires.

One aspect of the present invention is a thermoelectric conversionmodule illustrated in FIG. 8A, which has an insulator 19 between thep-type thermoelectric conversion elements 12 and the n-typethermoelectric conversion elements 13. By including the insulator 19,the strength of the p-type thermoelectric conversion elements 12 and then-type thermoelectric conversion elements 13 can be reinforced, enablingthe durability to be improved. From the viewpoint of the level ofreinforcement, the insulator 19 preferably covers the entire sidesurface of the p-type thermoelectric conversion elements 12 and then-type thermoelectric conversion elements 13.

One aspect of the present invention is a thermoelectric conversionmodule illustrated in FIG. 8B, which does not have an insulator 19between the p-type thermoelectric conversion elements 12 and the n-typethermoelectric conversion elements 13. By not having the insulator 19,external loss of heat from the p-type thermoelectric conversion elements12 and the n-type thermoelectric conversion elements 13 is suppressed,and as a result, a high thermoelectromotive force can be obtained.Examples of the above insulating plate and the above insulator includeceramic plates of alumina or aluminum nitride or the like.

As described above, because the p-type thermoelectric conversionelements and the n-type thermoelectric conversion elements in thethermoelectric conversion module are connected electrically in series,the output from the thermoelectric conversion module is a valuesubstantially equal to the product of the output of one thermoelectricconversion element and the number of thermoelectric conversion elementsused. In other words, in order to increase the output from thethermoelectric conversion module, increasing the output of eachthermoelectric conversion element, or increasing the number ofthermoelectric conversion elements used is effective.

As described above, because the p-type thermoelectric conversionelements and the n-type thermoelectric conversion elements are connectedin an alternating arrangement, the relationship between the number (P)of p-type thermoelectric conversion elements and the number (N) ofn-type thermoelectric conversion elements in the thermoelectricconversion module is typically P=N+1, P=N, or N=P+1 (wherein N and P areintegers of 1 or greater).

The sum of the number of p-type thermoelectric conversion elements andthe number of n-type thermoelectric conversion elements in thethermoelectric conversion module may be adjusted appropriately dependingon conditions such as the size of the thermoelectric conversion moduleand the electromotive force required. In one aspect of the presentinvention, the sum of the number of p-type thermoelectric conversionelements and the number of n-type thermoelectric conversion elements inthe thermoelectric conversion module is preferably from 50 to 1,000,more preferably from 50 to 500, and even more preferably from 50 to 250.

The compound and the thermoelectric conversion material of the presentinvention, in addition to use in conventional thermoelectric conversiondevices using the Seebeck effect, can also be employed in thermoelectricconversion devices that use a thermomagnetic effect such as the Nernsteffect, the Righi-Leduc effect and the Maggi-Righi-Leduc effect, orthermoelectric conversion devices that use a spin Seebeck effect basedon a spin pumping effect or the inverse spin Hall effect.

[Thermoelectric Conversion Properties of Thermoelectric ConversionMaterial]

Indicators that can be used to show the thermoelectric conversioncharacteristics of the thermoelectric conversion material include zT,which is an indicator of the thermal efficiency, and the output factor(power factor), which is an indicator of the output. By using theSeebeck coefficient α [V/K], the resistivity ρ[Ωm], and the thermalconductivity κ [W/(mK)] which are the thermoelectric conversionproperties at a temperature T, zT can be calculated using the formula(2) shown above, and the power factor can be calculated using theformula (3) shown above.

The present invention is able to provide a thermoelectric conversionmaterial having a low carrier density. It is surmised that this isbecause the carrier density, which tends to increase when germaniumtelluride is doped with silver and antimony, can be reduced byadditional doping with copper and bismuth, meaning the carrier densitycan be optimized.

Because the thermoelectric conversion material of the present inventionhas low carrier density, a thermoelectric conversion material having ahigh zT value can be provided. Further, a thermoelectric conversionmaterial having a high power factor can be provided. Accordingly, byusing the thermoelectric conversion material of the present invention, athermoelectric conversion module having high thermal efficiency and highoutput characteristics can be produced.

In particular, by using the thermoelectric conversion material of thepresent invention, the power factor and zT can be increased not onlywithin a specific temperature range, but across a range from roomtemperature to 500° C. Accordingly, by using the thermoelectricconversion material of the present invention, thermoelectric conversionof comparatively high output and high efficiency can be achieved evenfrom low temperatures near room temperature. Even when fluctuations inthe temperature of the waste heat occur due to the operating state of adevice, comparatively high output and high efficiency can still beachieved.

<Method for Producing Compound>

There are no particular limitations on the method used for producing thecompound of the present invention. A preferred embodiment of the methodfor producing the compound of the present invention is described below.

Aspects of the present invention include the methods for producing acompound described below.

<1> A method for producing a compound containing at least germanium,tellurium, bismuth, copper, antimony and silver as constituent elements,the method including:

a step of mixing raw materials containing at least germanium, tellurium,bismuth, copper, antimony and silver, and melting the raw materials byheating at 720° C. or higher, and

a step of quenching the melt by immersion in a liquid of less than 50°C.

<2> A method for producing a compound containing at least germanium,tellurium, bismuth, copper, antimony and silver as constituent elements,the method including:

a step of powdering materials containing at least germanium, tellurium,bismuth, copper, antimony and silver, and

a step of sintering the powdered material at 400° C. or higher using aplasma sintering method, by passing a pulsed electric current throughthe powder while compressing the powder, thereby generating electricaldischarges within the powder and heating the powder.

<3> A method for producing a compound containing at least germanium,tellurium, bismuth, copper, antimony and silver as constituent elements,the method including,

a step of mixing raw materials containing at least germanium, tellurium,bismuth, copper, antimony and silver, and melting the raw materials byheating at 720° C. or higher, and

a step of quenching the melt by immersion in a liquid of less than 50°C. to obtain an ingot,

a step of powdering the ingot, and

a step of sintering the powdered material at 400° C. or higher using aplasma sintering method, by passing a pulsed electric current throughthe powder while compressing the powder, thereby generating electricaldischarges within the powder and heating the powder.

First Embodiment

A first embodiment of the method for producing the compound is a methodfor producing a compound containing at least germanium, tellurium,bismuth, copper, antimony and silver as constituent elements, andincludes a step of mixing raw materials containing at least germanium,tellurium, bismuth, copper, antimony and silver, and melting the rawmaterials by heating at 720° C. or higher (hereinafter referred to as“the melting step”), and a step of quenching the melt by immersion in aliquid of less than 50° C. (hereinafter referred to as “the quenchingstep”).

[Melting Step]

The melting step is a step of mixing raw materials containing at leastgermanium, tellurium, bismuth, copper, antimony and silver, and meltingthe raw materials by heating at 720° C. or higher.

The maximum temperature during the heating in this embodiment is 720° C.or higher. In order to melt the raw materials and improve theuniformity, heating at a temperature of 940° C. or higher that is higherthan the melting point of germanium is preferable, and heating at atemperature of 1,090° C. or higher that is higher than the melting pointof copper is more preferable.

In another aspect of the present invention, the maximum temperatureduring the heating in this embodiment is preferably from 720° C. to1,500° C., and in order to melt the raw materials and improve theuniformity, heating at a temperature of 940° C. to 1,500° C. is morepreferable, and heating at a temperature of 1,090° C. to 1,500° C. iseven more preferable.

The rate of temperature increase during heating in this embodiment ispreferably from 0.5 to 1,000° C./minute, and more preferably from 1 to200° C./minute.

Further, heating is preferably performed for 0.1 to 100 hours, and morepreferably for 0.5 to 20 hours.

There are no particular limitations on the melting method, and variousmethods may be used.

Examples of the melting method include heating using a resistanceheating element, high-frequency-induced degradation, arc melting, andelectron beam melting.

Examples of the crucible used include graphite, alumina, and coldcrucibles, which may be used as appropriate in accordance with theheating method.

In order to prevent the raw materials described above and the ingotdescribed below from deteriorating due to contact with the air orliquids, the raw materials and the ingot are heated, and then quenchedin the subsequent quenching step, in an inert atmosphere of argon,nitrogen, or a vacuum or the like. The raw materials may be packed intoan inert atmosphere ampule in advance, and then subjected to heating andcooling.

[Quenching Step]

The quenching step is performed after the aforementioned melting step ofmelting the mixture of raw materials containing germanium, tellurium,bismuth, copper, antimony and silver at a temperature of at least 720°C. that represents the melting point of germanium telluride, and is astep of quenching the melt by immersion in a liquid of less than 50° C.,thus obtaining an ingot containing germanium, tellurium, bismuth,copper, antimony and silver.

In the quenching step in this embodiment, it is desirable that thetemperature of the melt is cooled rapidly from a temperature at least ashigh as the melting point of the melt to a temperature of 100° C. orlower, and cooling to 100° C. or lower is preferably performed within 10minutes. Cooling is more preferably performed within 5 minutes, and evenmore preferably within 1 minute.

Examples of materials that may be used as the liquid mentioned aboveinclude materials that are liquid at temperatures of 100° C. or lower,such as water, ice water, solutions containing water as the maincomponent, liquid nitrogen and liquid air. In terms of being inexpensiveand offering a high degree of safety, water, ice water, solutionscontaining water as the main component, and mixtures thereof arepreferable.

This embodiment has the quenching step described above. It is thoughtthat by including the quenching step, bismuth, copper, antimony andsilver can be doped in a supersaturated state into the germaniumtelluride. As a result, it is surmised that by using this embodiment,low elemental segregation can be achieved in which the longest axis ofthe bismuth crystals, copper crystals, antimony crystals or silvercrystals that are ubiquitous within the compound is less than 2.0 μm.

In contrast, in a conventional air cooling method, it is surmised thatthe bismuth, copper, antimony or silver do not dissolve in the germaniumtelluride matrix, resulting in precipitation of the bismuth, copper,antimony or silver that exceeds the saturated composition. If bismuth,copper, antimony or silver precipitate, then the longest axis of thebismuth crystals, copper crystals, antimony crystals or silver crystalswill become 2.0 μm or greater.

Second Embodiment

A second embodiment of the method for producing the compound is a methodfor producing a compound containing at least germanium, tellurium,bismuth, copper, antimony and silver as constituent elements, andincludes a step of powdering materials containing germanium, tellurium,bismuth, copper, antimony and silver (hereinafter referred to as “thepowdering step”), and a step of performing sintering at 400° C. orhigher using a plasma sintering method (hereinafter referred to as “theplasma sintering step”).

[Powdering Step]

The powdering step is a step of powdering materials containinggermanium, tellurium, bismuth, copper, antimony and silver.

In the powdering step, an ingot containing germanium, tellurium,bismuth, copper, antimony and silver is produced, and the ingot is thencrushed and powdered using a ball mill or the like. The melting step andquenching step described above can be employed as the method forproducing the ingot. Although there are no particular limitations on theparticle size of the powdered fine particles, the particle size ispreferably not more than 150 μm.

In another aspect of the present invention, the particle size of thepowdered fine particles is preferably from 0.1 μm to 150 μm, and morepreferably from 0.5 μm to 100 μm.

[Plasma Sintering Step]

The plasma sintering step is a step of performing sintering at 400° C.or higher using a plasma sintering method. A discharge plasma sinteringstep in this embodiment is a step of passing a pulsed electric currentthrough the powder obtained in the powdering step by powdering compoundscontaining germanium, tellurium, bismuth, copper, antimony and silver,while compressing the powder, thereby generating electrical dischargeswithin the powder, and heating and sintering the sample to obtain acompound.

When the electric current is halted in the discharge plasma sinteringstep, heating stops and the sample cools rapidly. In order to preventuneven distribution of the composition and enhance the thermoelectricconversion characteristics of the compound, it is preferable thatheating is performed at a prescribed temperature, and the discharge isthen halted and the sample cooled.

In order to prevent the above compound containing germanium, tellurium,bismuth, copper, antimony and silver from deteriorating due to contactwith air, the discharge plasma sintering step is preferably performed inan inert atmosphere of argon, nitrogen, or a vacuum or the like.

The compression in the discharge plasma sintering step is performedwithin a range from 0 to 100 MPa. In order to obtain a compound of highdensity, the compression is preferably at least 10 MPa, and morepreferably 30 MPa or greater. In other words, in order to obtain acompound of high density, the compression in the discharge plasmasintering step is preferably from 10 MPa to 100 MPa, and more preferablyfrom 30 MPa to 100 MPa.

The heating temperature in the discharge plasma sintering step ispreferably significantly lower than the temperature at which thecompound containing germanium, tellurium, bismuth, copper, antimony andsilver starts to melt, and is more preferably not higher than 650° C. Atemperature of 600° C. or lower is more preferable. On the other hand,in order to promote sintering, heating is preferably performed at acomparatively high temperature, and the temperature is preferably atleast 400° C. A temperature of 500° C. or higher is more preferable. Inother words, the heating temperature in the discharge plasma sinteringstep is preferably from 400° C. to 650° C., and more preferably from500° C. to 600° C.

The heating in the discharge plasma sintering step is preferablyperformed for 0.01 to 25 hours, and more preferably 0.05 to 10 hours.

This embodiment has the discharge plasma sintering step described above.It is thought that by including the discharge plasma sintering step, thesample is cooled rapidly, thus enabling supersaturated bismuth, copper,antimony and silver to be doped into the germanium telluride.Accordingly, it is surmised that by using the second embodiment of thecompound of the present invention, low elemental segregation can beachieved in which the longest axis of the bismuth crystals, coppercrystals, antimony crystals or silver crystals that are ubiquitouswithin the compound is less than 2.0 μm.

Third Embodiment

A third embodiment of the method for producing the compound is a methodfor producing a compound containing at least germanium, tellurium,bismuth, copper, antimony and silver as constituent elements, andincludes a melting step of mixing raw materials containing at leastgermanium, tellurium, bismuth, copper, antimony and silver, and meltingthe raw materials by heating at 720° C. or higher, a quenching step ofquenching the melt by immersion in a liquid of less than 50° C. toobtain an ingot, a step of powdering the ingot, and a step of sinteringthe powder at 400° C. or higher using a plasma sintering method.

In this embodiment, descriptions relating to the melting step, thequenching step, the powdering step and the plasma sintering step are thesame as the descriptions provided above for the melting step and thequenching step in the first embodiment, and the powdering step and theplasma sintering step in the second embodiment.

Because this embodiment combines the quenching step and the plasmasintering step, it is thought that satisfactory dissolution can beachieved without precipitation of bismuth crystals, copper crystals,antimony crystals or silver crystals.

EXAMPLES

The present invention is described below in further detail usingexamples. Analyses of the characteristics and structures of compoundswere conducted using the following methods.

1. Seebeck Coefficient

Measurements of the Seebeck coefficient α [V/K] and the resistivityρ[Ωm] were performed using a thermoelectric conversion characteristicsevaluation device ZEM-3 (manufactured by ULVAC-RIKO, Inc.). Samples usedin the measurements were cut using a diamond cutter. A typical shape forthe sample was height: 6.3 mm, width: 4 mm, and depth: 1.7 mm. R-typethermocouples used for temperature measurement and voltage measurementwere secured to the sample so as to contact the sample at 2.7 mmintervals along the height direction. The sample was heated to aprescribed temperature in a helium gas atmosphere (0.01 MPa). Moreover,by heating one end of the sample, a temperature difference was producedalong the height direction of the sample. The temperature difference(ΔT) and the voltage difference (ΔV) between the R-type thermocoupleswere measured. The temperature difference (ΔT) was set to values withina range from 1 to 10° C. The voltage difference (ΔV) was measured whenthree different temperature differences (ΔT) were imparted to thesample. The value of the Seebeck coefficient α was calculated from theslope of the voltage difference (ΔV) relative to the temperaturedifference (ΔT).

2. Resistivity

In the measurement of the Seebeck coefficient using the thermoelectricconversion characteristics evaluation device ZEM-3 (manufactured byULVAC-RIKO, Inc.), the resistivity was measured using a DC four-terminalmethod.

3. Power Factor

The power factor [W/(mK²)] was calculated using the above formula (3),using the measured Seebeck coefficient α [V/K] and resistivity ρ[Ωm].

4. Thermal Conductivity

The thermal conductivity κ [W/(mK)] was calculated from the thermaldiffusivity λ[m²/s], the heat capacity C_(p)[J/g] and the densityd[g/m³], using a formula (4) shown below.

[Numerical Formula 4]κ=λ×C _(p) ×d  (4)

Measurements of the thermal diffusivity λ, the heat capacity C_(p) andthe density d were performed using the same sample. The sample used forthe measurements was cut using a diamond cutter. A typical shape for thesample was 4 mm×4 mm×0.5 mm.

5. Thermal Diffusivity

A laser flash analyzer LFA457 MicroFlash (manufactured byNETZSCH-Geratebau GmbH) was used for measurement of the thermaldiffusivity λ. At the time of measurement, the surface of the sample wascoated black with a carbon spray “Graphite 33” (manufactured by CRCIndustries Europe BVBA).

6. Heat Capacity

An EXSTAR DSC 7020 (manufactured by SII NanoTechnology Inc.) was usedfor measurement of the heat capacity C_(p).

7. Density

A density measurement kit (manufactured by Mettler-Toledo InternationalInc.) was used for measurement of the density d, with the measurementperformed at 20° C. using the Archimedes method, with water as theliquid.

8. Thermoelectric Figure of Merit z

The thermoelectric figure of merit z [1/K] was calculated from theSeebeck coefficient α [V/K] at an absolute temperature T as zT, theresistivity ρ[Ωm], and the thermal conductivity κ [W/(mK)], using theabove formula (2).

9. Crystal Structure Analysis

Analysis of the crystal structure was performed using a powder X-raydiffractometer RINT TTR-III (manufactured by Rigaku Corporation), byperforming a powder X-ray diffraction measurement under the followingconditions to obtain a powder X-ray diffraction pattern. The intensityratio I(Ge)/I(GeTe) between the maximum intensity I(GeTe) of the XRDpeak attributable to germanium telluride and the maximum intensity I(Ge)of the XRD peak attributable to germanium metal was calculated from thepowder X-ray diffraction pattern.

Measurement apparatus: Powder X-ray diffractometer RINT TTR-III(manufactured by Rigaku Corporation)

X-ray generator: CuKα ray source, voltage: 30 kV, current: 400 mA

Slit: Variable slit (focusing method), slit width: 2 mm

X-ray detector: scintillation counter

Measurement range: diffraction angle 2θ=10° to 90°

Sample preparation: powdered by mortar grinding

Sample stage: special-purpose glass substrate, thickness: 0.2 mm

10. Analysis of Composition Distribution

Analysis of the composition distribution of the compound was performedusing a scanning electron microscope JEOL ISM-6701F (manufactured byJEOL Ltd.) fitted with an energy dispersive X-ray spectrometer BrukerAXS XFlash Detector 5030 (manufactured by Bruker AXS GmbH), and acomposition distribution was obtained under the following conditions.The surface of the sample was polished in advance to a mirror surface,and was then subjected to finishing by wet polishing with a lapping filmsheet 1 μm (manufactured by 3M Corporation).

SEM: JEOL ISM-6701F (manufactured by JEOL Ltd.), accelerating voltage:15 kV, current: 20 μA

EDX: XFlash Detector 5030 (manufactured by Bruker AXS GmbH) Analysissoftware: QUANTAX 200 (manufactured by Bruker AXS GmbH)

11. Analysis of Crystal Domains

Analysis of crystal domains was performed using a transmission electronmicroscope JEOL JEM2800 (manufactured by JEOL Ltd.) fitted with anelectron probe having a diameter of 1 nm, and a TEM image of a surfacecorresponding with the ab plane of the R3m structure was obtained usingthe STEM mode at an accelerating voltage of 200 kV. The sample wasappropriately exfoliated in advance, and an ion mill Gatan PIPS(manufactured by Gatan, Inc.) was used to perform finishing at roomtemperature using an Ar ion beam having an accelerating voltage of 2 kV.

12. Carrier Density

Five-terminal Hall measurement using a physical property measurementsystem PPMS (manufactured by Quantum Design, Inc.) and a special-purposeDC resistance sample pack was used for measurement of the carrierdensity p[cm⁻³]. A typical shape for the sample was length: 6 mm×depth:2 mm×thickness: 0.4 mm.

The Hall measurement was performed by adjusting the sample to aprescribed temperature, and then applying a magnetic field within arange from −5 T to 5 T perpendicularly to the sample surface andmeasuring the Hall resistance. The Hall coefficient was calculated fromthe slope of the Hall resistance relative to the magnetic field, and thecarrier density was then calculated from the Hall coefficient.

Example 1

In Example 1, a compound containing germanium, tellurium, bismuth,copper, antimony and silver was obtained via a quenching step (1) and adischarge plasma sintering step (2).

(1) Quenching Step

For raw materials, germanium (manufactured by Furuuchi ChemicalCorporation, powder 100 mesh, purity: at least 99.999%), tellurium(manufactured by Osaka Asahi Metal Mfg. Co., Ltd., granular, 6NS-2Grade), bismuth (manufactured by Osaka Asahi Metal Mfg. Co., Ltd.,granular, 6N Grade), copper (manufactured by Kojundo Chemical LaboratoryCo., Ltd., powder 850 μm pass, purity: at least 99.999%), antimony(manufactured by Osaka Asahi Metal Mfg. Co., Ltd., granular, 6N GradeS-2), and silver (manufactured by Furuuchi Chemical Corporation, powder300 mesh, purity: at least 99.99%) were used.

In Example 1, the raw materials were weighed to achieve a composition ofthe chemical formula Ge_(1+a-b-c-d-e)Bi_(b)Cu_(c)Sb_(d)Ag_(e)Te in whicha=0.00, b=0.04, c=0.04, d=0.13 and e=0.13, and a mixture was obtainedusing an agate mortar. Subsequently, 2.5 g of the mixture was placed ina quartz ampule (inner diameter: 5 mm, outer diameter: 6 mm), and theampule was sealed under reduced pressure of 3×10⁻⁴ Pa or lower. Thequartz ampule was then heated to 950° C. in an electric furnace, and themixture was melted. The temperature was raised to 950° C. at a rate of5° C./minute, and then held at 950° C. for 5 hours.

In the quenching step, the quartz ampule was removed from the 950° C.electric furnace, and immediately immersed in room temperature water. Atthis time, the mixture inside the quartz ampule was quenched, andsolidified rapidly from the melted state to form an ingot. Cooling fromthe melted state at 950° C. to 100° C. or lower was achieved within oneminute. The ingot was then recovered from the quartz ampule.

(2) Discharge Plasma Sintering Step

In the discharge plasma sintering step, a discharge plasma sinteringapparatus Doctor Sinter Lab SPS-511S (manufactured by Fuji ElectronicIndustrial Co., Ltd.) was used. The ingot obtained in the quenching stepwas powdered by mortar grinding. The powder was then packed in aspecial-purpose carbon die, and a compound was obtained by performingdischarge plasma sintering under the following conditions. The heatingwas performed for 10 minutes.

Apparatus: Doctor Sinter Lab SPS-511S (manufactured by Fuji ElectronicIndustrial Co., Ltd.)

Sample: powder 2.5 g

Die: special-purpose carbon die, inner diameter 10 mmø

Atmosphere: argon 0.05 MPa

Pressure: 40 MPa (3.1 kN)

Heating: 550° C., 10 minutes

The composition, carrier density and composition segregation of thecompound of Example 1 are shown in Table 1.

(Crystal Structure Analysis)

In a powder X-ray diffraction pattern for the compound of Example 1,only peaks attributable to germanium telluride were observed. No peaksattributable to germanium metal were observed.

(Composition Segregation)

The composition distribution of the compound of Example 1 is shown inFIG. 2. The longest axes of segregation of bismuth, copper, antimony andsilver were all less than 0.2 μm.

(Carrier Density)

The carrier density at 10 K in Example 1 was 5.3×10²⁰ cm⁻³.

(Thermoelectric Conversion Characteristics)

The temperature dependence of thermoelectric conversion properties ofthe compound of Example 1 is shown in FIG. 5. Thermoelectric conversioncharacteristics of the compound of Example 1 including the power factorand zT are shown in Table 2, and the temperature dependence of thethermoelectric conversion properties is shown in Table 3.

Example 2

With the exception of altering the composition, a compound of Example 2was obtained via the same quenching step (1) and discharge plasmasintering step (2) as Example 1.

(1) Quenching Step

In Example 2, with the exception that the raw materials were weighed toachieve a composition of the chemical formulaGe_(1+a-b-c-d-e)Bi_(b)Cu_(c)Sb_(d)Ag_(e)Te in which a=0.02, b=0.04,c=0.02, d=0.13 and e=0.13, the same quenching step as Example 1 wasperformed to obtain an ingot.

(2) Discharge Plasma Sintering Step

With the exception of using the ingot of Example 2, the same dischargeplasma sintering step as Example 1 was performed, thus obtaining acompound.

The composition, carrier density and composition segregation of thecompound of Example 2 are shown in Table 1.

(Crystal Structure Analysis)

In a powder X-ray diffraction pattern for the compound of Example 2,only peaks attributable to germanium telluride were observed. No peaksattributable to germanium metal were observed.

(Composition Segregation)

The composition distribution of the compound of Example 2 is shown inFIG. 3. The longest axes of segregation of bismuth, copper, antimony andsilver were all less than 0.2 μm.

(Carrier Density)

The carrier density at 10 K in Example 2 was 4.7×10²⁰ cm⁻³.

(Thermoelectric Conversion Characteristics)

The temperature dependence of thermoelectric conversion properties ofthe compound of Example 2 is shown in FIG. 5. Thermoelectric conversioncharacteristics of the compound of Example 2 including the power factorand zT are shown in Table 2, and the temperature dependence of thethermoelectric conversion properties is shown in Table 3.

Comparative Example 1

In Comparative Example 1, a compound containing germanium, tellurium,antimony and silver was obtained by performing the same quenching step(1) and discharge plasma sintering step (2) as Example 1. ComparativeExample 1 differs from Examples 1 and 2 in that no bismuth or copper wasincluded in the composition.

(1) Quenching Step

In Comparative Example 1, with the exception of using a composition ofthe chemical formula Ge_(1+a-b-c-d-e)Bi_(b)Cu_(c)Sb_(d)Ag_(e)Te in whicha=0.00, b=0.00, c=0.00, d=0.13 and e=0.13, the same quenching step asExample 1 was performed to obtain an ingot.

(2) Discharge Plasma Sintering Step

With the exception of using the ingot of Comparative Example 1, the samedischarge plasma sintering step as Example 1 was performed, thusobtaining a compound.

The composition, carrier density and composition segregation of thecompound of Comparative Example 1 are shown in Table 1.

(Crystal Structure Analysis)

In a powder X-ray diffraction pattern for the compound of ComparativeExample 1, only peaks attributable to germanium telluride were observed.No peaks attributable to germanium metal were observed.

(Composition Segregation)

The composition distribution of the compound of Comparative Example 1 isshown in FIG. 4. The longest axes of antimony and silver segregationwere both less than 0.2 μm.

(Carrier Density)

The carrier density at 10 K in Comparative Example 1 was 1.8×10²¹ cm⁻³.

(Thermoelectric Conversion Characteristics)

The temperature dependence of thermoelectric conversion properties ofthe compound of Comparative Example 1 is shown in FIG. 5. Thermoelectricconversion characteristics of the compound of Comparative Example 1including the power factor and zT are shown in Table 2, and thetemperature dependence of the thermoelectric conversion properties isshown in Table 3.

The compositions and material properties of the compounds of Examples 1and 2 and Comparative Example 1 are summarized in Table 1.

The carrier densities of the compounds containing germanium, tellurium,bismuth, copper, antimony and silver obtained in Examples 1 and 2 werelower than the carrier density of the compound containing germanium,tellurium, antimony and silver obtained in Comparative Example 1.

The composition segregation in each of the compounds of Examples 1 and 2and Comparative Example 1 was less than 0.2 μm for bismuth, copper,antimony and silver. In the powder X-ray diffraction patterns of thecompounds of Example 1 1 and 2 and Comparative Example 1, only peaksattributable to germanium telluride were observed, and no peaksattributable to germanium metal were observed.

The thermoelectric conversion characteristics including the power factorand zT for the compounds of Examples 1 and 2 and Comparative Example 1are compared in FIG. 5, and summarized in Table 2.

The power factor and zT values for the Example 1 were higher than thoseof Comparative Example 1 in a range from 25° C. to 500° C. Thetemperature dependence of the thermoelectric conversion characteristics(power factor and zT) for Example 1 was smaller than that forComparative Example 1. In other words, in Example 1, a compound wasobtained which not only exhibited comparatively high thermoelectricconversion characteristics at high temperatures, but because thetemperature dependence was small, also had high thermoelectricconversion characteristics at low temperatures.

The power factor of Example 2 exhibited a higher value than that ofComparative Example 1 in a range from 25° C. to 350° C., and in a rangefrom 350° C. to 500° C., the power factor values for Example 2 andComparative Example 1 were substantially equal. The zT value of Example2 was higher than that of Comparative Example 1 in a range from 25° C.to 500° C. The temperature dependence of the thermoelectric conversioncharacteristics (power factor and zT) for Example 2 was smaller thanthat for Comparative Example 1. In other words, in Example 2, a compoundwas obtained which not only exhibited comparatively high thermoelectricconversion characteristics at high temperatures, but because thetemperature dependence was small, also had high thermoelectricconversion characteristics at low temperatures.

TABLE 1 Ge_(1+a−b−c−d−e)Bi_(b)Cu_(c)Sb_(d)Ag_(e)Te Carrier densityComposition segregation [μm] No. a b c d e [cm⁻³] Bi Cu Sb Ag Example 10.00 0.04 0.04 0.13 0.13 5.3 × 10²⁰ <0.2 <0.2 <0.2 <0.2 Example 2 0.020.04 0.02 0.13 0.13 4.7 × 10²⁰ <0.2 <0.2 <0.2 <0.2 Comparative 0.00 0.000.00 0.13 0.13 1.8 × 10²¹ — — <0.2 <0.2 Example 1

TABLE 2 Power factor zT [μW/(cmK²)] [—] 200° 300° 400° 500° 200° 300°400° 500° No. C. C. C. C. C. C. C. C. Example 1 21 26 26 27 0.83 1.441.32 1.60 Example 2 20 24 25 24 0.76 1.13 1.38 1.52 Comparative 17 23 2626 0.46 0.77 1.14 1.35 Example 1

TABLE 3 Power factor zT P.F.(500° C.)/ zT(500° C.)/ P.F.(300° C.)zT(300° C.) Example 1 1.03 1.11 Example 2 1.00 1.36 Comparative Example1 1.13 1.75

INDUSTRIAL APPLICABILITY

The compound of the present invention exhibits excellent thermoelectricconversion characteristics even in the high-temperature region near 500°C., and can therefore be applied in various fields, includingvehicle-mounted applications.

DESCRIPTION OF REFERENCE SIGNS

-   11: Thermoelectric conversion element-   12: p-type thermoelectric conversion element-   13: n-type thermoelectric conversion element-   14: Metal electrode-   15: First metal electrode-   16: Second metal electrode-   17: Third metal electrode-   18: Insulating plate-   19: Insulator-   20: Thermoelectric conversion module-   21: Thermoelectric conversion device-   21′: Thermoelectric conversion device-   31: Lead wire-   32: Lead wire-   41: Heat source-   42: Heat radiating plate-   43: Electrode (negative electrode)-   44: Electrode (positive electrode)-   43′: Electrode (negative electrode)-   44′: Electrode (positive electrode)-   45: External load

The invention claimed is:
 1. A compound comprising at least germanium,tellurium, bismuth, copper, antimony, silver, and one or more optionalelements other than germanium, tellurium, bismuth, copper, antimony, andsilver as constituent elements, wherein an amount of each optionalelement is not more than 0.2 mol per 1 mol of tellurium.
 2. The compoundaccording to claim 1, having a carrier density of not more than 1.0×10²¹cm⁻³.
 3. The compound according to claim 1, represented by a chemicalformula Ge_(1+a-b-c-d-e)Bi_(b)Cu_(c)Sb_(d)Ag_(e)Te (wherein−0.05≤a≤0.10, 0<b≤0.10, 0<c≤0.10, 0<d≤0.20, and 0<e≤0.20).
 4. Thecompound according to claim 1, wherein an intensity ratio I(Ge)/I(GeTe)between a maximum intensity I(GeTe) of an XRD peak attributable togermanium telluride and a maximum intensity I(Ge) of an XRD peakattributable to germanium metal is not more than 0.025.
 5. The compoundaccording to claim 1, wherein a longest axis of bismuth, copper,antimony or silver, which are ubiquitous within the compound, is lessthan 2.0 μm.
 6. A thermoelectric conversion material comprising thecompound according to claim
 1. 7. The compound according to claim 1,wherein the compound is a germanium telluride compound doped withbismuth, copper, antimony, and silver.
 8. A compound consisting ofgermanium, tellurium, bismuth, copper, antimony, and silver asconstituent elements.
 9. The compound according to claim 8, having acarrier density of not more than 1.0×10²¹ cm⁻³.
 10. The compoundaccording to claim 8, represented by a chemical formulaGe_(1+a-b-c-d-e)Bi_(b)Cu_(c)Sb_(d)Ag_(e)Te (wherein −0.05≤a≤0.10,0<b≤0.10, 0<c≤0.10, 0<d≤0.20, and 0<e≤0.20).
 11. The compound accordingto claim 8, wherein an intensity ratio I(Ge)/I(GeTe) between a maximumintensity I(GeTe) of an XRD peak attributable to germanium telluride anda maximum intensity I(Ge) of an XRD peak attributable to germanium metalis not more than 0.025.
 12. The compound according to claim 8, wherein alongest axis of bismuth, copper, antimony or silver, which areubiquitous within the compound, is less than 2.0 μm.
 13. Athermoelectric conversion material comprising the compound according toclaim
 8. 14. The compound according to claim 8, wherein the compound isa germanium telluride compound doped with bismuth, copper, antimony, andsilver.